Depression and anxiety disorders are among the leading causes of morbidity, mortality, and disability in the United States, and are highly associated with exposure to stress. Impaired serotonin neurotransmission appears to be a central mechanism inducing depressive and anxiety symptoms. Previous studies have suggested that corticotropin releasing factor (CRF) receptor activation in the dorsal raphe, a major source of serotonin to the forebrain, is a critical mechanism underlying stress effects on serotonergic systems. However, the effects of CRF on dorsal raphe vary depending on dose, location within the nucleus, and receptor specificity. The origin of CRF projections to the dorsal raphe, and their potential to be regulated by dorsal raphe outputs is also unknown. Nor are the roles in stress response of the several neuropeptides increasingly recognized as co-transmitters with serotonin that may be involved in the effects of chronic stress. I will use a serotonergic cell line, RN46A-B14 to model the dose response and second messenger effects of CRF receptor activation on serotonergic neurons, helping to elucidate the direct effects of CRF there. Using a novel retrograde gene transfer system, canine adenovirus-2, 1 will determine the origins of CRF projection to the various subregions of the dorsal raphe, suspected to be the central nucleus of the amygdala and bed nucleus stria terminalis. I will also determine reciprocal projections of the dorsal raphe to these regions, delineating the circuitry that could underlie intrinsic neuromodulation, a phenomena that may underlie some of the long lasting behavioral alterations associated with stress. Finally, I will also produce a conditional knockout Tph2 transgenic mouse and determine the ability of a canine adenovirus-Cre recombinase vector to specifically alter serotonergic transmission in defined pathways. This methodology will allow me to separate the role of serotonin from the role of peptide co-transmitters in behavior. These studies will help develop technologies to understand the role of CRF projections to the dorsal raphe, and the role of dorsal raphe outputs in modulating circuits that underlie behavior related to anxiety and depression. The public health importance of this basic research is substantial. Understanding the mechanisms by which stress is communicated opens opportunities for intervening in this process. Given the role of stress in the development and recurrence of many psychiatric disorders, the ability to disrupt the communication of stress context would be a boon for both the treatment and prevention of many types of mental illness, including depression and anxiety disorders. This research will help provide the information necessary for the rational development of such interventions.